Jordi Paps: From its humble beginnings, life has infected the entire planet with infinite pretty forms. The origin of life is the oldest biological event, so aged that it has left behind no clear evidence beyond the existence of life itself.
This leaves many open questions, one of the most intriguing being how many times life has magically emerged from inanimate matter.
Did all life on Earth evolve just once, or are different living things made of different materials? The question of how tough it is for life to emerge is engaging—not least because it might shed some lightweight on the likelihood of finding life on other planets.
The origin of life is a central question in newfangled biology, and probably the most hard to study. This event occurred four billion years ago and occurred at a molecular level – meaning there is little fossil evidence left.
Many living beginnings have been suggested, from unsavory primordial soups to outer space. But the current scientific consensus is that life emerged from nonliving molecules in a natural process called abiogenesis, most likely in the darkness of deep-sea hydrothermal vents. But if life emerged once, why not more times?
What is abiogenesis?
Scientists have proposed various successive steps in abiogenesis. We know that Earth was luxurious in several chemicals, such as amino acids, a type of molecule called nucleotides or sugars, which are the building blocks of life.
Laboratory experiments like the iconic Miller-Urey experiment have shown how these compounds can form naturally in conditions similar to those on the early Earth. Some of these compounds may also have arrived on meteorites.
These straightforward molecules then combine to form more complicated ones, such as fats, proteins, or nucleic acids. Importantly, nucleic acids—such as double-stranded DNA or its single-stranded cousin RNA—can store the information needed to build other molecules. DNA is more stable than RNA, but in contrast, RNA can be part of chemical reactions in which a compound makes copies of itself—self-replication.
The “RNA world” hypothesis proposes that early in life, RNA may have been used as material for genes and replication, before DNA and proteins emerged.
Once an information system can make copies of itself, natural selection begins. Some of the fresh copies of these molecules (which some call “genes”) will have errors or mutations, and some of these fresh mutations will improve the molecules’ ability to replicate.
So over time, there will be more copies of these mutants than other molecules, and some of them will accumulate more mutations, making them become even faster and more habitual, and so on.
Eventually, these molecules probably evolved a lipid (fat) boundary separating the internal and external environments of the organism, forming protocells. Protocells could better concentrate and organize the molecules needed for biochemical reactions, providing a closed and competent metabolism.
Life in circles?
Abiogenesis may have occurred more than once. Earth may have spawned self-replicating molecules several times, or perhaps early life simply consisted of many different self-replicating RNA molecules, of independent origin, competing for the same building blocks, for thousands or millions of years. Unfortunately, because of the archaic and microscopic nature of the process, we may never know.
Many laboratory experiments have successfully reproduced different stages of abiogenesis, indicating that they could have occurred more than once. However, we are not certain that they actually did occur in the past.
A related question might be whether fresh life is arising by abiogenesis as you read this. But that is highly unlikely. The early Earth was devoid of life, and the physical and chemical conditions were very different. Today, if ideal conditions existed somewhere on the planet for the emergence of fresh self-replicating molecules, they would be immediately eaten up by existing life.
We know that all living things in existence descend from one common last universal common ancestor of life (also known as LUCA). If there were other ancestors, they left no descendants.
Key evidence supports the existence of LUCA. All life on Earth uses the same genetic code, namely the correspondence between the nucleotides in DNA known as A, T, C, and G—and the amino acid they code for in proteins. For example, the three-nucleotide sequence ATG always corresponds to the amino acid methionine.
Theoretically, however, there could be more variations in the genetic code between species. But all life on Earth uses the same code with a few minor changes in some lines.
Biochemical pathways, such as those used to metabolize food, also support the existence of LUCA; many independent pathways could have evolved in different ancestors, but some (such as those used to metabolize sugars) are common to all living organisms. Similarly, hundreds of identical genes are present in different living things, which can only be explained by their being inherited from LUCA.
My favorite support for LUCA comes from the Tree of Life. Independent analyses, some using anatomy, metabolism, or genetic sequences, have revealed a hierarchical pattern of kinship that can be visualized as a tree.
This shows that we are more closely related to chimpanzees than to any other organisms living on Earth. Chimpanzees and we are more closely related to gorillas, and together with orangutans and so on.
You can pick any organism, from lettuce in a salad to bacteria in bioactive yogurt, and if you go back in time far enough, you will have a common ancestor. This is not a metaphor, but a scientific fact.
It is one of the most amazing concepts in science, the Darwinian unity of life. If you are reading this, you are here because of an unbroken chain of reproductive events stretching back billions of years.
While the thought of life continuing to emerge on our planet or anywhere else is exhilarating, what is even more exhilarating is the idea that we are related to all living things on this planet.
Jordi Paps, Senior Lecturer, Department of Biological Sciences, University of Bristol, University of Bristol
This article was reprinted from Conversation under the Creative Commons license. Read original article.
Image Source: Pixabay.com
